Method of blowing furnances and system for the carrying out of the method

ABSTRACT

A method has been provided for the operation of a blast furnace wherein air is introduced in the blast furnace through a tuyere zone and an air flow maintained at high velocity. This method comprises of introducing the supplemental fuel into said tuyere zone and admixing the same with the air flowing through the tuyere zone; maintaining at least one lower velocity turbulent zone in said tuyere zone, said lower velocity being in respect to the velocity of the flowing air and supplemental fuel admixture, said lower velocity turbulent zone initiating and sustaining the combustion of said fuel; and controlling the amount of supplemental fuel consumed in the tuyere by varying the distance of the turbulent zone in respect to the zone at which the air flow exists from said tuyere. Apparatus illustrating various flow retarding devices and thus flame source and fuel combustion control means also have been disclosed.

Unite inventors Appl. No.

Filed Patented Assignee Priority Louis Duthion Paris:

Arsene Marzouvanlian. Plaisir; Robert Augustin Chaube, Maule, all of. France 782.860

Dec. 11, 1968 Aug. 3, 1971 La Societe De Wendel Paris, France Dec. 13, 1967. Apr. 30, 1968 lFrance 132,105 and 150,195

METHOD OF BLOWING FURNANCES AND SYSTEM FOR THE CARRYING OUT 01- THE METHOD 11 Claims, 27 Drawing Figs.

[56] References Cited UNlTED STATES PATENTS 982,244 1/1911 Case .i 266/41 1,870,511 8/1932 Hopkins 266/41 Primary Examiner-Gerald A. Dost Aimrney-Curtis. Morris & Safford ABSTRACT: A method has been provided for the operation ofa blast furnace wherein air is introduced in the blast furnace through a tuyere zone and an air flow maintained at high velocity. This method comprises of introducing the supplemental fuel into said tuyere zone and admixing the same with the air flowing through the tuyere zone; maintaining at least one lower velocity turbulent zone in said tuyere zone, said lower velocity being in respect to the velocity of the flowing air and supplemental fuel admixture, said lower velocity turbulent zone initiating and sustaining the combustion of said fuel; and controlling the amount of supplemental fuel consumed in the tuyere by varying the distance of the turbulent zone in respect to the zone at which the air flow exists from said tuyere. Apparatus illustrating various flow retarding devices and thus flame source and fuel combustion control means also have been disclosed.

Patented Aug. 3, 1971 596,894

6 Sheets-Sheet 1 Patented Aug. 3, 1971 3 596594 6 Sheets-Sheet a Patented Aug. 3, 1971 3,596,894

6 Sheets-Sheet Patented Aug. 3, 1971 6 Sheets-Sheet 4 Patented Aug. 3, 1971 3,596,894

6 Sheets-Sheet &

Patented Aug. 3, 1971 e Sheets-Sheet 6 [00/3 DU TH/O/V METHOD OF BLOWING FURNANCES AND SYSTEM FOR THE CARRYING OUT OF THE METHOD In numerous shaft furnaces, and in particular blast furnaces, a part of the solid fuel has already been replaced by injections of hydrocarbon products such as petroleum oil or so-ealled fuel oil, natural gas, or coke oven gas, in the zone of the tuyeres discharging into the lower part of the shaft furnaces. Such injections or charges make it possible to economize on solid fuel which is generally relatively expensive. Furthermore, these charges favor the rise of the temperatures of the blowing gas, commonly referred to as blast" and make it possible to increase the overoxygenation thereof.

These effects have a beneficial influence on the efficiency of the furnace. However, such supplemental addition are frequently limited by the fact that starting at a certain amount, the injected, supplemental fuel burns more and more incompletely as the supplemented proportion increases.

One of the purposes of the present invention is to assure the complete combustion of the supplemented fuel at the very outlet of the tuyeres or to complete the combustion in the turbulent zones which are established in the vicinity of the downstream end of the said tuyeres.

The main object of the present invention is a method by which at least the original source of the flame in the tuyere is stabilized by establishing the ignition at a suitable, possibly adjustable, point in order to protect the tuyere, first, against the influence ofthe combustion; and second, to establish the frac tion of the supplemental fuel which is burned in the tuyere itself, which fraction may now amount to the entire amount of said fuel.

Another object of the invention is the system or apparatus for the carrying out of this method. This apparatus comprises means for causing a gas to flow through the tuyere towards the furnace, means for introducing anadditional or supplemental fuel into the gas flowing through the tuyere, means for accelerating the flow of gas in the tuyere in the vicinity of the outlet orifice for the supplemental or additional fuel and means for igniting the mixture of gas and supplemental fuel at a given place.

The means for discharging the supplemental fuel into the gases may either be orifices provided in a wall in contact with the blast gases or a conduit which protrudes into the tuyere parallel or slightly inclined (in the direction with the flow of the gases) to a diametral plane ofthe tuyere.

The means for accelerating the flow of gas in the vicinity of the outlet orifices of supplemental fuel consists of either a physical constriction or neck or of an aerodynamic constriction.

The physical constriction may be made in known manner, for instance, by imparting a suitable shape to the walls of the tuyere or by providing in the tuyere a divergent-convergent body with solidwalls which is preferably coaxial to the tuyere. The latter can be provided with means for discharging the supplemental fuel.

To the extent that the high temperatures which prevail at the outlet of the tuyeresare not sufficient for the flame to be ignited by itself, means which can be contemplated for igniting the mixture of gas and supplemental fuel are either a displacement towards the furnace in which better ignition conditions prevail or any other auxiliary device which leaves the stabilizing means stationary, for instance pyrotechnical or electrical ignition.

The means for fixing the point of origin or source of the flame at a given spot consists of at least one solid obstacle or at least one fluid obstacle located downstream of the discharge orifice of the supplemental fuel.

In order to protect the walls of the tuyere from the heat, an additional wall can be provided therein spaced from the tuyere towards the inside thereof and extending from the downstream end ofthe said tuyere at least up to the means for the discharge of the addition fuel. One can then cool this additional wall by a circulation of fluid in the space which said wall defines withthewall ofthe tuyere.

In similar manner, the solid obstacles may be hollow and cooled in the manner which has just been indicated.

Finally, and particularly when the axial speed of flow reaches or extends Mach 0.4 in the tuyere, a chamber followed by an expansion nozzle is provided therein.

As a further refinement in the operation of blast furnaces, it has been found during tests that the obstructions are subjected to extremely high temperatures due to the radiation of the heat of the blast furnace. It is also necessary to use costly materials, for instance tungsten, for these obstructions unless an effective cooling system can be provided. Moreover, it has been found that slag raises during the blast furnace operation, the latter solidifies in contact with the cooled walls of the tuyere and obstructs it. The presence: of obstructions inthe center of the tuyere makes it hardly possible to have recourse to the conventional method of freeing the tuyeres with a device known as a rake."

Consequently, the additional refinement consists in the use of a solid obstruction which does not have these drawbacks, i.e., which obstruction permits the igniting of the gaseous mixture and the fixing of the point of origin of the flame at a given place in accordance with the above-disclosed method, but which obstruction does not thereby encumber the center of the tuyere and which can be cooled by the cooling system of the tuyere itself.

The following description, read with reference to the accompanying drawings, which have been given by way of illustration and not of limitation, will make it clear how the invention can be carried out, the details appearing both from the drawings and from the specification constituting, of course, a part of the invention.

In the drawings, the apparatus is illustrated by various means with special emphasis on the fixing of the point of origin of flame in the tuyere itself, and the combustion characteristics of the supplemental fuel imparted to it by the depicted configurations, and wherein:

FIGS, 1 to 10 represent in cross section various embodiments of the invention along the lines I-l, IlII...X-X, respectively, of FIGS. la to 10a;

FIGS. la to on and 9a are end view of the embodiments shown in FIGS. 1 to 6 and 9, respectively;

FIGS. 7a, 8a and 10a are sections along the lines X-X, Y-Y and Z-Z of FIGS. 7, 8, and 10, respectively;

FIG. 11 is a reproduction of FIG. l in which symbols of geometrical and physical magnitudes are added;

FIG. 12 is a schematic view in longitudinal section ofa blast tuyere ofa furnace fed with an addition fuel and provided with an obstacle incorporated in the wall of the tuyere;

FIG. 13 is a cross section through the tuyere along the line "-11 of FIG. 12;

FIG. 14 is a view similar to FIG, 13 of a variant of an obstacle in accordance with the invention;

FIGS. 15 and 116 are transverse and longitudinal sections through an obstacle of conical shape and its cooling system along the line IV-lV of FIG. 16 and along the line c-V of FIG. 15, respectively;

FIG. I7 is a view along the arrow F of the obstacle shown in FIG. 16.

FIG. I shows a tuyere ll whose downstream end 2 discharges into a furnace (not shown). The tuyere I has a constriction or a neck 3 and is provided with at least one injector 4 whose discharge orifice 5 is located in the diametral plane of the neck or its immediate vicinity downstream of the latter. A solid constriction 6 which is substantially coaxial to the tuyere and flared from upstream direction to downstream direction is held by shaped arms 7 at a position located between the neck 3 and the end 2.

The arms 7 are shaped so as not to produce a wake which might give rise to and maintain a secondary or parasitic flame. The operation is as follows:

The speed of the gases passing through the tuyere increases through the constriction 3 which favors the mixing of these gases with the supplemental or. addition fuel discharged at 5 and the discharge itself. The resultant mixture is ignited by one of the means indicated above, for instance in the vicinity of the downstream face 8 of the obstruction 6, primarily due to the radiation of the material contained in the furnace.

As a result, the flame remains fixed in position due to the recycling to the face 8 of the hot gases supplied by the flame itself. which gases maintain and support the flame at said place.

The eddies of gas can be formed at least in part of gases coming from the furnace.

This zone constitutes the hottest zone of the flow and therefore maintains the point of ignition at this place.

In the embodiment shown in FIG. 2, the single solid obstruction 6 is replaced by a plurality of obstructions 6a to 6e of a shape similar to that of the obstruction 6. held in position respectively by shaped arms such as 7a and 7d.

In the embodiment of FIG. 3. the obstructions 8a and 8b are annular and substantially coaxial to the tuyere l to which they are connected by shaped arms such as 70 and 7d. Along an axial plane, each of the obstructions is flared outwardly in downstream direction.

In the embodiment shown in FIG. 4, the obstruction is toroidal and is connected to the tuyere by conduits 10a and 10b which extend into the walls of the tuyere and are transversed, as is also the obstruction 9, by a cooling fluid.

The introduction of the supplemental fuel is effected, as in the previous embodiments, via an injector II which, in FIG. 4, is shown shifted 90 with respect to the injectors 4 of FIGS. 1 to 3.

COnduits such as 10a and 10b can also be combined with the obstructions 8a and 8b of FIG. 3 which, in that case, are hollow and cooled by the circulation of the fluid.

In the embodiment shown in FIG. 5, a complementary wall 12 is arranged within the tuyere. This wall is substantially coaxial to the tuyere and together with the wall 2a defines an annular space 13. The wall 12 extends axially at least up to the discharge orifice 5 s0-that the mixing of the gas and the fuel take place on the inside of the complementary wall 12. The latter is cooled by the circulating in the space 13 ofa cooling fluid which, in the present case, consists of a fraction of the gases passing through the tuyere. It can be seen that upstream of the constriction, these gases are much less hot than they are downstream thereof due to the combustion of their mixture with the addition fuel.

In the embodiment shown in FIG. 6, the constriction does not consist ofa deformation of the wall 2a but a solid body 14 which is divergent-convergent from upstream to downstream and is connected by a rod 15 to the obstruction 6.

The advantage of this embodiment is that the assembly of the members l4, l5, 6, 7a to 7c can be put in place from upstream to downstream, while in the preceding embodiments, the obstructions are advantageously introduced into the tuyere or into the complementary wall 12 from the downstream end. Of course, when the assembly of obstructions and their arms described in the preceding embodiments can be folded for the putting of them in place or have a suitable shape, this assembly can also be introduced from the upstream end.

In the embodiment shown in FIG. 7, there can be noted the sealing jacket I2 connected by means, not shown in the drawing, to the wall of the tuyere. The obstruction in this variant is aerodynamic. that is to say consists ofjets 0f fluids such as 15, exiting from orifices such as 16 provided in the wall 12 and connected to conduits such as 17. The turbulent zone 8 is formed in the wake of the jets such as 15 and the turbulent zone such as 8 is heated by radiation from the furnace to a temperature sufficient to cause the igniting ofthe mixture.

The fluids are discharged from the orifices 16 under pressure and may be air, steam or any inert gas or vapor.

One can also discharge through these orifices a certain amount of supplemental fuels in gaseous or vapor form. If this supplement fuel is a liquid or a pulverulent solid, it is fed by a gas or a vapor under pressure such as those mentioned above which serves as vehicle for it and withthe fuel forms and additional combustion supporting mixture. Hence. these fuels are designated supplemental or addition fuels. v

In the embodiment of FIG. 8, the addition fuel i s' discharged completely through the conduits I7 and it is the wake of the 5 jets thus obtained which constitutes both the aerodynamic construction and the obstruction. g

The ignition can take place spontaneously when the temperature of the gases, due to the radiation of the material contained in the furnace is sufficiently high. Otherwise, an ignition device known per se, is used.

In the embodiment of FIG. 9, the obstruction I8 is a bulb which is divergent-convergent from upstream to downstream and has a downstream face 19.

The supplemental fuel is discharged through at least one orifice such as 20 located in or preferably in the downstream vicinity of the diametral plane of the constriction 21. This orifice is fed through a conduit such as 22, a manifold 23 and tubes such as 24. The conduits 22 are advantageously provided in the arms 25.

The embodiment of FIG. 10 is more particularly adapted to a flow of blast gases, the speed of which is at least equal to Mach 0.4. Downstream of the constriction 3, the tuyere comprises a chamber 26 at the inlet to which the obstruction 6 is arranged. This chamber, which makes it possible substantially to reduce the speed offlow, is followed by an expansion nozzle 27 the dimensions of which are such as to obtain the desired velocity of discharge of the blast gases into the furnace.

FIG. 11 shows an embodiment which has already been described in connection with FIG. 1-. In this figure there have been entered symbols of physical magnitudes such as velocity of flow of the blast gases, for instance, and symbols of the geometric dimensions. THese symbols have the following meaning:

li s the distance from the downstream end obstruction to the downstream end or exit of the tuyere.

L is the distance from the orifice for the discharge of the addition fuel to the downstream end of the tuyere.

Ve is the axial velocity of flow of the blast gases.

Vn is the normal velocity of combustion at the flame front.

R is half the hydraulic diameter of the tuyere.

r is half the external hydraulic diameter of the obstruction.

his the spray distance.

The studies carried out by the applicants have shown that the obstruction should be placed at a distance lfrom the downstream end of the tuyere such that:

These studies have also shown that the best operating condi- To summarize the above, the apparatus which is illustrated in the above figures shows that the flame of addition fuel originates in the blast tuyere and is stabilized by ignition at a suitable place which may be adjustable. This method of operating the blast furnace makes it possible to assure the complete combustion of the addition or supplemental fuel at the very outlet of the tuyeres or to complete the combustion in the turbulent zones which are established in the vicinity of the downstream of the said tuyeres. As a result of this method, the amount of supplemental fuel can be increased and a costly fuel normally required can thus be replaced by another, cheaper fuel. The efficiency of the furnace is also increased since it is possible to increase the temperature of the blast and its concomitant overoxygenation.

Thus, the apparatus of the invention comprises, in combination, means for causing a gaseous fluid to pass through the tuyere towards the furnace (generally compressors), means for discharging the addition fuel into the gas passing through the tuyere (injection tubes or orifices), means for accelerating the flow of gas in the tuyere in the vicinity of the discharge orifree of the addition fuel (physical or aerodynamic constrictions) and finally means for igniting the mixture of gas and addition fuel and fixing the point of origin of the flame at a specified location. These last-mentioned means are the solid obstructions of various shapes illustrated in the figures which are placed within the tuyere.

Returning now to the above-mentioned refinement in the operation of the blast furnace and the novel obstructions alluded to above, the novel apparatus is discussed in light of FIGS. 12 to 17 which figures will be considered in greater detail below after the following disclosure of the general principles associated with the apparatus depicted in these figures.

The refinement type of obstruction in accordance with the invention is formed ofa protuberance of the wall ofthe tuyere itself which is integral with the inner ring thereof and cooled by the same means as cool the tuyere, the major part of the outlet section of the tuyere thus remaining free and accessible for freeing with a rake if needed. Numerous variants of ob structions can be conceived in accordance with the present refinement, but for each obstruction, such dimensions must be provided that the obstruction to the flow of the gas stream is sufficient to cause the separating thereof from the wall so that there can be produced in this separation zone a turbulent flow of low linear velocity of the gaseous flow in which the flame is stabilized. The dimensions of the obstruction must also be such that the stream of gas recirculates or comes back to the wall so as to prevent the separation zone being filled with gas intruding from outside of the combustion zone. Finally, it is necessary that the dimensions and the position of the obstruction, with respect to the discharge plane of the tuyere, permit the gaseous mixture to ignite in the separation zone by the mere radiation from the heat of the furnace.

The obstructions in accordance with the invention may have the most varied shapes, though, keeping in mind the es sential requirements of flame source and. flame maintenance and control. Each shape is characterized by the profile and the cross section of the obstruction. One particularly advantageous shape for the obstruction in accordance with the present invention consists of a sector of a cylindrical-ogival body, the cylindrical portion being located downstream of the ogival portion; more generally, with the obstruction terminat ing in a face perpendicular to the axis of the tuyere.

In the present specification herein it shall be understood by sector, the solid obtained by intersection of the inner surface of the tuyere (which has a circular cross section) by any surface, which surface may comprise, in the advantageous form of the obstruction mentioned above, an attack portion, also referred to as ogival," with generatrices which are not parallel to the axis of the tuyere, followed by a cylindrical portion, having generatrices which are parallel to the axis. The cone angle of the ogival portion, its length measured along the axis of the tuyere, the length of the cylindrical portion measured along this same axis, as well as the distance from the downstream base of the cylinder to the section of the nose of the tuyere are selected as a function of the speed of the blast in the tuyere in such a manner that the stream ofgas is separated from the wall of the obstruction at the downstream base of the cylinder returns to the wall of the tuyere in the vicinity of the nose thereof, thus, permitting the ignition ofthe gases in the separation zone due to the heat radiated by the furnace. It has been observed that the cone angle of the ogival portion should be between l and 30 and that the surface of the sector of the base of the cylinder forming the obstruction should not exceed 30 percent of the total section ofthe tuyere. A variant of this type of obstruction is obtained by breaking off the edges of the obstruction; in this case, a better structure of the flame is obtained. The edges of the obstruction may have different inclinations, and the cross section of the obstruction may, for instance, be trapezoidal, rectangular. triangular, etc.

When the obstruction is rather close to the outlet plane of the tuyere, it is important to have a cylindrical portion which makes apart possible to obtain recirculation eddies of the gases. Without this cylindrical portion there is danger of producing merely a deviation of the jet without a recirculation zone.

In order to obtain the ignition of the gaseous mixture, the placing of an electric resistor is possible in a tube fastened to the bottom of the curvature of the tuyere, said tube passing through the wall of the tuyere at the level of the obstruction in the cooling conduit and exiting into the tuyere downstream of the obstruction. Such a tube can also serve for measuring devices in the separation zone (thermometer, pressure gauge or the like).

The cooling of the obstruction is effected by a liquid, generally water, which circulates by forced circulation in the cooling circuit. This circuit may be common to the tuyere and to the obstructions or be separated into two portions, one of which is intended to cool the tuyere and the other to cool the obstruction. This latter manner of cooling is adopted when the temperature of the surface of the obstruction under the operating conditions ofthe tuyere is very high.

Referring now to FIGS. 12, i3 and 14 which illustrate the refinement in the concept of flame control and flame stabilization, the downstream end ofthe tuyere is formed ofa pipe 121 on which there is fastened a part 311 formed of two shells, between which water flows in the annular zone 41. The inner shell of part 31 has a protuberance 511 which is integral with it and is cooled by forced circulation of water, the entrance of which is shown schematically at m and the outlet at 71. Furthermore the tuyere is equipped with an injection or fuel introduction tube 811 and a tube 91 which passes through the annular cooling zone and discharges into the tuyere downstream ofthe obstacle in the separation zone 104.

When the blast blows in the tuyere in the direction indicated by the arrows llll, it produces a fuel mixture with the fuel injected through the tube 81 and which then encounters the obstacle 51. The gaseous stream separates from the obstacle at I12 and then returns to the wall of the tuyere at 113, thus, creating a turbulent or separation zone 101, where the linear speed of the gases is low and where the mixture can ignite and the flame be stabilized; to assist in the ignition of the mixture an electric resistor can be arranged within the tube 91. The latter can also be used for measurements of temperature or removal of samples of gas.

In the variant shown in FIG. 14, the obstacle 51 has been made flush on its edges at 51a so as to improve the stability of the flame.

In the embodiment shown in FIGS. l5, l6 and 17, the obstacle 114, which is a portion ofa hollow cylindrical-ogival solid, is cooled by a conduit system which is specific to it, the water introduced at 116 to cool the obstacle 114 emerging again at 117 independently of the cooling water of the tuyere itselfwhich flows in the annular zone 118,

It goes without saying that the invention is not limited to the described embodiments but it also is coextensive to those embodiments which can be realized by the use of equivalent technical means.

We claim:

ii. A tuyere for a blast furnace having means for introduction of supplemental fuel and a convergent-divergent section formed by the walls thereof comprising in combination in a section thereof, downstream of the means for introduction of supplemental fuel and upstream from the downstream, furnace exit end of the tuyere, a turbulence introducing obstruction or constriction means, a section of said means defining an initial flame point for the supplemental fuel, the initial flame point defining section of said constriction means, in reference to the furnace exit of tuyere being located at a distance based on amount of supplemental fuel burned in the tuyere.

2. The tuyere according to claim ll wherein the obstruction or constriction means is an axial flow constriction means.

3. The tuyere according to claim 2 wherein the axial flow constriction means consist of a number of individual constrictions of axial and concentric solid, conical annuli attached to the walls of the tuyere by holding means.

4. The tuyere according to claim 2 wherein the axial flow constriction means consist of solidcones supported by holding means attached to the walls of the tuyere.

5. THe tuyere according to claim I wherein the constriction means is internally cooled.

6 The tuyere according to claim I and wherein a complementary. circumferentially and longitudinally extending wall suspended from the tuyeres wall is within the tuyere at a distance providing air flow between the same and the tuyeres wall.

7. The tuyere according to claim 1 and wherein the constriction is defined by supplemental fuel introduced into the tuyere through circumferentially spaced apart orifice means.

8. The tuyere according to claim 1 wherein the constriction means is a constriction means having a divergent-convergent section protruding axially and upstream from the flame point defining constriction means.

9. The tuyere according to claim 1 wherein a convergent divergent section is formed therein from the walls thereof upstream from the constriction means.

10. The tuyere according to claim 1 wherein the position of obstruction means in the tuyere are defined by the relationship tionship OslSHt-m Ve/Vn v l-( Vn/Ve) in which: l is the distance from the downstream end of the obstruction to the downstream exit end ofthe tuyere;

L is the distance from the discharge orifice of the addition fuel to the downstream exit end of the tuyere;

A is the spray distance;

Ve is the axial velocity of flow of the blast gases;

Vn is the normal combustion velocity at the flame front;

R is half the hydraulic diameter of the tuyere;

r is half the outer hydraulic diameter of the obstruction. 

1. A tuyere for a blast furnace having means for introduction of supplemental fuel and a convergent-divergent section formed by the walls thereof comprising in combination in a section thereof, downstream of the means for introduction of supplemental fuel and upstream from the downstream, furnace exit end of the tuyere, a turbulence introducing obstruction or constriction means, a section of said means defining an initial flame point for the supplemental fuel, the initial flame point defining section of said constriction means, in reference to the furnace exit of tuyere being located at a distance based on amount of supplemental fuel burned in the tuyere.
 2. The tuyere according to claim 1 wherein the obstruction or constriction means is an axial flow constriction means.
 3. The tuyere according to claim 2 wherein the axial flow constriction means consist of a number of individual constrictions of axial and concentric solid, conical annuli attached to the walls of the tuyere by holding means.
 4. The tuyere according to claim 2 wherein the axial flow constriction means consist of solid cones supported by holding means attached to the walls of the tuyere.
 5. THe tuyere according to claim 1 wherein the constriction means is internally cooled.
 6. The tuyere according to claim 1 and wherein a complementary, circumferentially , and longitudinally extending wall suspended from the tuyere''s wall is within the tuyere at a distance providing air flow between the same and the tuyeres wall.
 7. The tuyere according to claim 1 and wherein the constriction is defined by supplemental fuel introduced into the tuyere through circumferentially spaced apart orifice means.
 8. The tuyere according to claim 1 wherein the constriction means is a constriction means having a divergent-convergent section protruding axially and upstream from the flame point defining constriction means.
 9. The tuyere according to claim 1 wherein a convergent-divergent section is formed therein from the walls thereof upstream from the constriction means.
 10. The tuyere according to claim 1 wherein the position of obstruction means in the tuyere are defined by the relationship 0 1 L - lambda in which 1is the distance from the downstream end of the obstruction to the downstream exit of the tuyere; L is the distance from a discharge orifice of the supplemental fuel to the dowNstream exit of the tuyere; lambda is the spray distance.
 11. The tuyere according to claim 1 wherein the positions of the obstruction means in the tuyere are defined by the relationship 0 1 (R-r) Ve/Vn 1-(Vn/Ve)2 in which: 1 is the distance from the downstream end of the obstruction to the downstream exit end of the tuyere; L is the distance from the discharge orifice of the addition fuel to the downstream exit end of the tuyere; lambda is the spray distance; Ve is the axial velocity of flow of the blast gases; Vn is the normal combustion velocity at the flame front; R is half the hydraulic diameter of the tuyere; r is half the outer hydraulic diameter of the obstruction. 